Seasonal compartmentalisation of water in a grassland at 2600 m a.s.l

High Alpine catchments are precious water-resources since they act as natural storage reservoirs, storing water in the snow cover and in the subsurface and thereby providing water during the dry seasons. Thus, a deeper knowledge of the hydrological functioning of these systems is necessary, in particular to make climate change projections. The role of seasonality is crucial in these catchments that generally exhibit a snow-dominated hydro-climatic regime. Here we use high-frequency observations of stable isotopes of water to identify the seasonal origin of streamwater in a high-elevation Alpine catchment located in the Valle d’Aosta Region, Italy. We quantify the relative contribution of winter and summer precipitation reaching the stream through the Seasonal Origin Index (SOIQ), calculated using the δ18O values and the volumes of precipitation and streamflow. Highly negative SOIQ values are obtained suggesting that streamwater is mainly composed of winter precipitation. Conversely, the Seasonal Origin Index for evapotranspiration (SOIET), which can be directly inferred from SOIQ, returns a positive value reflecting that plants preferentially take up water deriving from summer precipitation. These findings allow us to develop a conceptual model of this Alpine system. This conceptual model suggests: - a deep infiltration component, mainly composed by snowmelt water, reaching the stream through a preferential flow. - a shallow infiltration component, predominantly represented by summer rainfall, that dominates the shallow soils and that is used by plants. Therefore, we presume a seasonal compartmentalisation of water in this high-elevation catchment. Nevertheless, a previous study in Switzerland revealed SOIQ ≈ 0 for the Allenbach and Dischmabach snow-dominated catchments, indicating that similar fractions of summer and winter precipitation become streamflow. This different result achieved in systems with an apparently similar functioning highlights the need for a deep insight into the flow paths governing high-elevation catchments and it opens the way for new challenges to understand the hydrological processes hidden behind this difference.